1. Field of the Invention
The invention relates to processes for the continuous and simultaneous conversion of carbon monoxide, hydrocarbons, and nitrogen oxides contained in hot gases, and in particular in hot exhaust gases, from an internal combustion engine.
2. Description of the Prior Art
Gaseous waste products resulting from the burning or combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. In recent years, with the ever-growing number of automobiles powered by internal combustion engines, the discharge of waste products therefrom has been of increasing concern, particularly in urban areas where the problem is more acute, and the control thereof has become exceedingly important. Of the various methods which have been proposed for converting the carbon monoxide, hydrocarbon and nitrogen oxide pollutants to innocuous products, the incorporation of a catalytic converter in the exhaust system holds the most promise of meeting the increasingly rigid standards established by the responsible governmental agencies.
In order to achieve a substantially simultaneous conversion of the carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ a catalyst in conjunction with a fuel-air ratio control means which functions in response to a feedback signal from an oxygen sensor in the engine exhaust gases. The fuel-air ratio control means is typically programmed to provide fuel and air to the engine in a ratio conducive to a near stoichiometric balance of oxidants and reductants in the hot exhaust gases at engine cruising conditions, and to a stoichiometric excess of reductants at engine idling and acceleration conditions. The result is that the composition of the gas with which the catalyst is contacted fluctuates almost constantly, such that conditions to which the catalyst is exposed are alternatively net reducing and net oxidizing. A catalyst for the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen must be capable of operating in such a dynamic environment.
The class of exhaust gas conversion catalysts herein contemplated, commonly referred to as three component control catalysts, must therefore function under variable conditions. Ideally, the catalyst should be capable of functioning under dynamic net oxidizing-net reducing conditions to catalyze the reaction of said pollutants with each other and/or any of the oxygen, hydrogen, carbon dioxide or water components which occur in hot exhaust gases fluctuating between a molar excess of oxidants and a molar excess of reductants. In particular, the catalyst should be capable of functioning during those more extended periods of fuel-rich operation, such as are encountered at engine idling and acceleration conditions, when the deficiency of oxidants in the exhaust gas becomes more acute. In other words, the catalyst should be capable of effecting the conversion of the otherwise oxidizable carbon monoxide and hydrocarbon pollutants in the absence of sufficient oxidants, such as oxygen and nitric oxide.
Catalytic composites comprising rhodium and platinum and/or palladium as the catalytic components have heretofore been proposed for the catalytic conversion of exhaust gases from an internal combustion engine. Frequently, the catalytic composite will further comprise a base metal component, typically nickel. While certain of the base metals are known to catalyze one or more of the various reactions which constitute the exhaust gas conversion process, they are in themselves substantially less effective, and in some cases ineffective, at the dynamic net oxidizing-net reducing conditions herein contemplated. Also, certain base metals demonstrate sharply decreased performance in the presence of sulfur. Other base metal components, although catalytically inert, are included in the catalytic composite for their contribution to physical and/or thermal stability (see U.S. Pat. Nos. 4,053,556; 3,140,148; 4,153,579; 4,171,287; and U.K. Pat. No. 1,405,405).